Thanks!
As someone who only did GCSE maths and did it back in 2011, I found the questions hard. I didn't revise at all for them though so I think as long as you are refreshing yourself on arithmetic and things like percentages, ratios, SDT, and wordy questions then you'll be fine. The example booklet they send out is so easy, the questions at Briefing are much much harder so don't use that as a gauge!
I know I've posted on a thread somewhere a link to the books I've used to revise for Main Board (I'm end of April). I got them off Amazon and the most useful numerical reasoning one is by Rob Williams. Its It's pink book, search numerical reasoning and you should find it!
Changing rooms wise: I didn't see any but there were toilets and a coat room where some people changed tshirts etc, so taking off layers there should be fine!
Good luck with Briefing, honestly don't stress about it! The point of it is so you know how much prep and what prep you need to do for Main Board